Nylon bags are frequently used in fire investigations to preserve samples such as fire debris for later analysis. They are considered to have very good resistance to vapour loss [1]. However, this does not mean that they hold onto all vapours forever, and this could make the difference between detecting accelerants or not, or mis-identifying the type of accelerant.

We showed in an earlier blog the type of profile seen for petrol (gasoline) vapours. Petrol and volatile solvents, such as naphthas are the most vulnerable to vapour loss because the molecules are very small and can diffuse through the polymer into the air.

We used bags from 2 different suppliers, adding 1µl of a synthetic mix of hydrocarbons in the gasoline boiling range to dry cellulose fabric, sealing the bag with a ‘swan-neck’ cable tie closure and encasing the bag with a second bag, also sealed with a swan-neck closure. The bags were then stored at ambient temperatures and the atmosphere inside each inner and outer bag tested after 20 days.

We used a synthetic mix in order to see if particular types of hydrocarbons migrated through preferentially.
This is the headspace GC-MS chromatogram of the synthetic mix (red chromatogram), showing C0-C3 aromatics (benzene through to trimethylbenzene) in the green chromatogram, alkanes from C5 pentane to C12 dodecane (purple chromatogram) and cycloalkanes and alkenes (C5 to C8) in the black chromatogram. The total aromatics content of the mix was 34%, similar to petrol.

Here are the chromatograms of the vapour from the inner and outer bags after 20 days ambient storage.

Nylon Bag 1

Nylon Bag 2

The red chromatogram is vapour from the Inner bag and the green chromatogram is for vapour sampled from the Outer bag.

After 20 days the levels of vapour in the inner bags were significantly lower than the vapour concentrations immediately after sealing the bags. The concentration of vapour in the outer bags was lower than for the inner bags and there appears to be a preferential loss of aromatic hydrocarbons. This could affect the interpretation of the analysis as the presence of the C1 and C2-benzenes in particular helps to define whether petrol (gasoline) is present or not.

In both the types of nylon bags used, volatile hydrocarbons permeated or diffused  through to the outer bag, and onwards to the atmosphere from the outer bag. But is this because of the inefficiency of the swan-neck closure, allowing volatiles to escape from the bag, or are the hydrocarbons permeating through the polymer bag itself ? [2]. We will be examining this in our next blog on this subject.

To summarise, it is best if analysis of fire debris for accelerants is carried out as soon after sample collection as possible. Contact SMS Analytical to discuss our rapid accelerant testing service.

References:

[1] E. Stauffer, JA Dolan, R Newman: Fire Debris Analysis, Academic Press, Elsevier, 2008

[2] Strijnik and Hong-You: Evaluation of the Effectiveness of Nylon Bags as Packaging for Fire Debris, 2004, Proceedings of 56th Annual Meeting of the American Academy of Forensic Sciences.

Here at SMS Analytical, we’ve been thinking about what makes our business attractive to our customers so we decided to present a list of reasons they return to us with more work:

Understanding The Problem.

Quick to grasp the details of problem that is being solved. With experience in a wide range of chemical ‘areas’ we are able to suggest analyses outside of the typical international and national standards or aternative techniques to overcome matrix interferences.

Quality

Attention to detail and methodology is key to being an analytical scientist. In this respect we have been refining our operations and have brought in changes as the result of obtaining our ISO9001 certification in 2016. All quotations and reporting is peer reviewed before sending to our customers and we seek to obtain feedback once the report has been read.

Agility

Proportionate use of analysis techniques which may be changed as the results from previous techniques are made. This means we would normally phone and email our client to discuss the preliminary results from the first analysis especially as it could be that we would advise not wasting time and money by analysing further using the techniques previously agreed in the quote. Instead, we may suggest alternative and more appropriate techniques, based on what we’ve discovered so far, or even recommend a stop at that point.

Turnaround Times

As a guideline, we normally would expect to deliver the customer’s report in about 10 working days, depending on the nature of the work and the techniques involved. For non routine analysis, this is a very competitive situation compared to many of the big laboratories out there.

Communication

Being a small company we are unhindered by layers of administration or red-tape and so can respond in a timely manner to customer queries such as pre quotation questions or post reporting questions.

Any additional thoughts about these would be welcome…